I. Field of the Invention
This invention relates generally to roller conveyors for carrying articles from a first location to a second location, and more particularly to slip roller conveyors with elongated rollers adapted to carry articles of a non-planar shape and without the need for a flat pallet to support the irregular-shaped articles during transport.
II. Discussion of the Prior Art
Many forms of conveyors are known in the art for moving articles along a production line from a first point to a second point. For example, prior art conveyors commonly comprise a frame having a pair of side rails held in parallel, spaced-apart relation and supporting a plurality of cylindrical rollers that are journaled for rotation between the side rails. If the conveyor is inclined to the horizontal, gravity may cause the conveyed products to move when the force of gravity exceeds the frictional resistance of the bearings used to journal the rollers. Alternatively, the rollers may be driven by an electric motor through a chain or belt that is made to engage the rollers.
One problem with positive drive roller conveyors of the prior art is that they can mar the article being transported in those instances where articles become backlogged on the conveyor because a product in-feed rate exceeds the rate at which the articles are removed from the conveyor.
One solution to this problem is afforded by my earlier U.S. Pat. Nos. 5,558,205 and 5,558,206, the teachings of which are hereby incorporated by reference. The conveyors described in those patents include rollers mounted on the opposed side rails that include a first roller element that is positively driven and that includes a stub shaft extending from a side surface of that first roller element. A second roller element is mounted in surrounding relation to the stub shaft of the first roller element with a clearance fit. The first roller element of the roller assemblies that is affixed to one of the rails is driven by a motor through an elongated, endless, toothed, timing belt. Rectangular pallets span the space between the second roller elements on each of the side rails and carry the product being conveyed. With this arrangement, the weight of the pallet plus the article being transported results in frictional engagement between the second roller element and the stub shafts on the first roller elements to move the pallets and products along the conveyor at a rate determined by the motor drive arrangement. However, in the event of a backlog condition, there is not enough friction between the first and second roller elements to move the backlogged products and the stub shafts of the first roller elements slip inside the second roller elements. Thus, the endless belt can continue to drive the first roller elements, but the second roller elements remain stationary until the backlog condition no longer prevails.
Slip roller conveyors of the type described and claimed in my earlier U.S. Pat. No. 5,558,206 are well suited for use in the electronics industry for transporting printed circuit boards along a desired path as pick-and-place robots are used to assemble electronic components on to the printed circuit boards. The conveyor would be designed such that the spacing between pairs of slip rollers affixed to the opposed side rails are generally equal to the width dimension of the printed circuit boards to be transported. The printed circuit boards, themselves being planar, ride smoothly on the slip rollers. However, for non-planar articles, it becomes necessary to place them on generally planar pallets which, themselves, are made to span the width dimension between roller pairs on the opposed side rails of the conveyor frame. This adds processing steps in that the empty pallets leaving the terminal end of the conveyor must be carried back to the conveyor's input end for reuse.
From what has been described, then, persons skilled in the art will appreciate that a need exists for a slip roller conveyor that can be used to transport irregularly-shaped articles but that do not require the use of planar pallets. The present invention fulfills that need.